As a few have mentioned, taking Duck Lake Road (MT-464) out of Browning is definitely the easiest way but not the only way. The current shortest (by 12 miles) and fastest (just by 4 minutes) way is to take the Starr School Road from Browning to US 89 and then US 89 into St. Mary. This route has many 25-45 mph curves and a 5 mile downgrade as you enter St. Mary but is very doable if you take your time and have excellent braking. To get to Starr School Road, you make a left at the Cenex station as you are leaving Browning. This will connect with US 89 north of the MT-49 junction.
I am not sure I would want to take our coach into St. Mary this way, even with exhaust brakes, but we did leave St Mary going the opposite direction towards Browning and enjoyed the ride. I bring this up as that is the preferred way to reach Browning from St. Mary if you are not driving or pulling your RV but just going into Browning for groceries.
Unless they have completely repaired the road, US 89 out of Browning is currently under construction and nothing but a mud path for six miles as you leave Browning. That is the way many GPSs will route you!
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